The present invention relates to a stainless steel sheet and a substrate on which a non-single crystalline semiconductor for solar cells is deposited and a resulting solar cell. The present invention is also concerned with a manufacturing method thereof.
A glass sheet, a stainless steel sheet or the like has been used as a substrate for a non-single crystalline semiconductor solar cell. The non-crystalline semiconductor shall be continuously deposited as a homogeneous layer of 1 .mu.m or thinner. In this regard, when a stainless steel sheet is used as such a substrate, it shall be finished to smoothly finished state.
However, an excessively smoothened surface returns incident light ray with specular reflection. The specular reflection means ineffective outgoing of incident light rays and puts limitation on increase of an energy conversion efficiency.
The energy conversion efficiency of a non-single crystalline semiconductor solar cell is increased by scattering and multiple reflection as well as by specular reflection of the incident light ray at the interface between the semiconductor and the substrate. The scattering and multiple reflection of the incident light ray is accelerated by changing a surface of the substrate to minutely rugged state. Such a rugged surface may be obtained by electrolytic etching, mechanical polishing, Ni plating etc. For instance, a Ni plating layer formed on a stainless steel sheet is conditioned to proper surface roughness, as disclosed in Japanese Patent Publication Nos. 5-13389 and 5-64870.
When a steel sheet used as a substrate is conditioned to rugged state by electrolytic etching, inclusions in the steel sheet are preferentially dissolved or dislodged so as to form numerous irregular pits. Due to the irregular pits, the treated surface becomes to uneven rugged state. Besides, the surface of the substrates is contaminated by adhesion of carbonaceous smuts.
In the case of mechanical polishing, a surface profile of a polished substrate is strongly affected and varied by a particle size of used abrasives, mixing ratio, pressure, etc., so that it is difficult to finish a substrate to uniformly rugged surface state. When excessively fine abrasives are used for polishing, dents on the substrate are easily filled with the abrasives, and the substrate can not be finished to properly rugged state. In this regard, it is not preferable to use too fine abrasives. Besides, the mechanical polishing makes a rugged surface so sharp to cause cracking or destruction of a semiconductor layer deposited thereon, resulting in a short circuit or malfunction of a solar cell.
In a Ni plating method for making a rugged surface, a stainless steel sheet is coated with a Ni layer at an adhesion amount of 4-50 g/m.sup.2 per single surface, for instance. In this case, a particle size of Ni grains at the surface of the Ni layer shall be controlled within a range of 0.01-1.5 .mu.m, and surface roughness shall be controlled within a range of R.sub.max 0.01-0.6 .mu.m. If the surface roughness exceeds R.sub.max 0.6 .mu.m, the surface profile of the substrate is too much complicated. Due to the complicated profile together with inclusion of electrolytically deposited grains, a non-single crystalline semiconductor layer uniform in thickness can not be continuously deposited on the Ni layer. On the other hand, if the surface roughness is less than R.sub.max 0.01 .mu.m, the effect of the Ni layer on an energy conversion efficiency is inferior.